1. Technical Field
The present invention relates to display devices.
2. Related Art
A display device that includes an imaging element forming an image of an object to be projected and can generate a stereoscopic image in the air as a real image by this imaging element is disclosed in WO 07/116,639 (Patent Document 1) and JP-A-2009-229905 (Patent Document 2).
The imaging element used in the display device described in Patent Document 1 includes a plurality of unit optical elements each being formed of a dihedral corner reflector having two orthogonal mirror surfaces. This imaging element has the function of forming a real image of an object to be projected which is placed in one space of an element plane as a mirror image in a position in the other space of the element plane such that the real image and the mirror image are symmetric with respect to the element plane. As a result, when a three-dimensional object is placed in one space of the element plane, a stereoscopic image is generated in a position in the other space of the element plane such that the three-dimensional object and the stereoscopic image are symmetric with respect to the element plane.
The display device described in Patent Document 2 is configured so that the imaging element having a dihedral corner reflector similar to that of Patent Document 1 is moved above a display by a driving unit in the direction vertical to an element plane. In this display device, by changing images which are displayed on the display in synchronization with the vertical movement of the imaging element, stereoscopic midair images are generated in a space above the imaging element.
In the display device of Patent Document 1, to generate a stationary stereoscopic image, it is necessary simply to put a stationary three-dimensional object below the imaging element. However, to obtain an image of an electronically-generated dynamic three-dimensional object, a complicated structure for creating an electronically dynamic three-dimensional object by rotating a light-emitting diode (LED) array, for example, is required.
Moreover, in the display device of Patent Document 2, to allow human eyes to observe a stereoscopic image, it is necessary to move the imaging element at a frequency (for example, at a frequency of about 50 Hz or higher) at which no flicker occurs. This makes it impossible to increase the imaging element excessively, limits the size of the imaging element, and eventually limits the size of stereoscopic midair images.
Furthermore, a large-scale drive unit is required to move the imaging element at high speed in the direction vertical to the element plane.